Having captained a fair bit, I think the most important thing is a coherent plan. Talk to your bowlers before the game, and have them tell you what they are trying to do. No point telling your best inswing bowler to try and bowl to 2 slips or something like that. If you have a clear plan (i.e, full and wide), you can set a relevant field to it. Further, talk to your bowlers about their preferred ends, preferred times to bowl to etc etc - if you can keep them as happy as possible, it's a major win for you.
I'd also make sure you don't do the typical club cricketer captain of ball chasing. It's important to back your bowlers when it comes to the field placing. Just because one ball has been flicked through vacant mid wicket, don't chase the ball and place a fielder there - it's difficult for a bowler to focus on constantly changing the ball they are meant to be bowling.
When batsman get settled, there's two things I like to do in T20. The first is as a captain, slow the game down. It's a frantic game T20, so sometimes a two minute conversation with your bowler, even if you're talking about what pint to have after the game, can slow the momentum down for the batsman, and give your bowler a chance to clear their head. Secondly, don't be afraid to mix bowlers up in one over spells, and think about small things like bowling off spinners to left handers, or not bowling off spinners to a right hander to a short leg side boundary. These things, as well as mixing your bowlers up, can have a big difference.
With regards to the final 5 overs of the fielding innings, I think what's most important (along with having a clear plan) is getting your best fielders in the most important positions. Usually this is straight and cow corner, but if for example you're playing on a short straight boundary and therefore are banging it into the wicket to get the batsman to hit square, have your best fielders in these positions.
Finally, a good spinner is like gold dust in T20. If you can utilise one correctly, and continue to support them when they get hit for the odd six, I think you get rewarded handsomely.
Just my thoughts,
CBD